Nitrogen 15–enriched Precambrian kerogen and hydrothermal systems

Abstract

Nitrogen isotope data are reported for kerogen from Precambrian black shales at prehnite‐pumpellyite to greenschist facies and for micas from postmetamorphic hydrothermal quartz vein systems. Nitrogen in micas was acquired from N‐bearing aqueous metamorphic hydrothermal fluids generated, from breakdown of sedimentary kerogen and dehydration of K‐silicates, at deeper crustal levels. The 2.7 Ga kerogens and hydrothermal K‐micas yield δ15N values of 15–24‰, compared to existing data of 2–6‰ in Phanerozoic counterparts. Paleoproterozoic equivalents have intermediate δ15N of 7–12‰, implying a secular decrease in δ15N of shale kerogen. The 15N‐enriched nitrogen in Archean shales and hydrothermal vein systems cannot be caused either by N isotopic shifts accompanying metamorphism or Rayleigh fractionation because premetamorphic and postmetamorphic samples from the same terrane are both enriched and lack covariation of δ15N with N, C/N ratios, or metamorphic grade. The magnitude of shifts during progressive metamorphism of sedimentary rocks in previous studies is constrained at <1‰ to greenschist and about 3‰ to amphibolite facies. Furthermore, 15N‐enriched values cannot stem from long‐term preferential diffusional loss of 14N as samples were selected from terranes where 40Ar/39Ar ages are within a few million years of concordant U‐Pb ages; nitrogen is structurally bound in micas, whereas Ar is not. It is possible that the 15N‐enriched values stem from a different N cycle in the Archean, with large biologically mediated fractionations, yet the magnitude of the fractionations observed exceeds any presently known, and chemoautotrophic communities tend to depleted values. Earlier results on Archean cherts show a range from −6 to 30‰. Given the temporal association of chert‐banded iron formation with mantle plumes, the range is consistent with mixing between mantle N2 of −5‰ and the enriched marine reservoir identified in this study. We attribute the secular trend from Archean kerogen of 15–24‰ to Phanerozoic kerogen averaging 3–4‰ to a secondary atmosphere derived from CI‐chondrite‐like material and comets with δ15N of 30–42‰. Shifts of δ15N to its present values of 0‰ can be accounted for by a combination of sequestration of atmospheric N2 into sedimentary rocks and mantle degassing of −5‰ mantle N2.